The recent breakthrough of direct reprogramming adult mouse and human fibroblasts toward a pluripotent state by introducing Oct3/4, Sox2, Klf4, and c-Myc, has brought new hope for the generation of patient- and disease- specific stem cells without the ethical issues associated with the derivation of human embryonic stem (ES) cells. However before induced pluripotent stem (iPS) cells can be applied for therapeutic applications, it is necessary to assess the ability of these cells to differentiate towards the desired cell type. For instance, skeletal myogenic progenitors are generated very inefficiently during in vitro differentiation of ES cells into embryoid bodies (EBs). This is due to the scarcity of paraxial mesoderm within EBs. We have recently demonstrated that Pax3 enables the generation of myogenic progenitors from differentiating ES cells that are endowed with the capacity to restore muscle function following their engraftment in mdx mice. Thus here we plan to examine the proof of principle that iPS cells may be used in the future for the treatment of DMD by combining the generation of iPS cells with our approach to derive myogenic progenitors by conditional expression of Pax7. Moreover, if one envisions translating iPS cells to the clinic, one has to demonstrate that functional myogenic progenitors can be successfully obtained from human ES cells. This will be assessed here by applying conditional expression of Pax7 to human ES cells with the goal to apply this knowledge to future studies involving human iPS cells obtained from patients with Duchene muscular dystrophy. PUBLIC HEALTH RELEVANCE: Embryonic stem (ES) cells and induced pluripotent stem (iPS) cells hold great promise for the treatment of degenerative diseases, however to date studies on their potential use in the treatment of muscular dystrophies have been hampered by the difficulty of differentiating ES cells into skeletal muscle progenitors. This application builds on a novel method we have developed to generate muscle progenitors from mouse ES cells. We have shown that such progenitors can be transplanted into normal injured, and dystrophic mice, where they contribute to muscle fiber regeneration, and improve muscle function after injury. In these studies, we will apply this approach to wild-type mouse iPS cells (Aim 1) as well as ex vivo genetically corrected dystrophic mouse iPS cells (Aim 2), thus assessing proof-of-principle to whether these cells are endowed with in vivo regenerative potential. In Aim 3, we will investigate the mechanisms controlling muscle differentiation in human ES cells with the goal to apply this knowledge to future studies involving human iPS cells obtained from patients with Duchenne muscular dystrophy.